Field of the Invention
The present invention generally relates to non-contact sensing of physiological functions and, more particularly, non-contact sensing of cardiovascular functions.
Description of the Related Art
Cardiac monitoring has been sufficiently demonstrated as a method for assessing wellness, performance, cognitive, and stress states in everyday, clinical, and mission environments. This monitoring is traditionally derived from one of two different modalities: electrocardiography (ECG) or photoplethysmography (PPG).
Electrocardiography measures electrical potentials produced by the depolarization and repolarization of the various muscles of the heart as they pump blood to the body. These electrical potentials propagate to the skin's surface where they may be measured using surface electrodes. Measures of cardiac activity derived from ECG have been demonstrated, in numerous studies, to be sensitive to physiological changes related to the demands of piloting aircraft. Cardiac measures of both heart rate and heart rate variability have also been shown to be sensitive to flight maneuvers and segments in an operational tactical airlift aircraft. These contemporary results were utilized to aid in a decision making process related to changes made to the cockpit systems in the aforementioned platforms. Additional studies focused on workload evaluation in current operational missions, a large portion of which are similar to the approach used in the tactical airlift aircraft study as related to the assessment of cardiac activity for objective measurement of stress and workload. ECG is often considered the “gold standard” for measuring cardiac activity in both clinical and research areas because it directly measures the aggregate electrical activity produced by the muscles pumping blood in the heart. Unfortunately, ECG requires electrodes to be affixed to the skin surface, costing time and resources, as well as a potential discomfort to the subject. It also may not be suitable for long term/repeated monitoring or with individuals with high skin sensitivities. Additionally, ECG may be susceptible to artifact under conditions of motion, which shake the ECG leads or disturb the electrode-skin interface. Moreover, these problems are also present in more modern “wearable” sensors that are popular in fitness, mobile health and wellness, and consumer electronics arenas.
Another monitoring methodology for observing cardiovascular performance is plethysmography. Plethysmography is a measurement of blood flow, or more specifically blood volume, in the peripheral vasculature. Contact photoplethysmography (PPG) is most common and utilizes (non-specific) light-based sources as a transduction medium. PPG takes advantage of well-known absorbent and reflective properties of tissue and blood as they vary with wavelength(s) of the transducer. The most common application of PPG is in pulse oximetry devices, which are usually secured to a finger, toe, or earlobe. Such devices measure arterial oxygen saturation and may also be capable of measuring pulse rate and pulse rate variability (which are often employed as surrogates of heart rate and heart rate variability). Unfortunately, traditional PPG sensors require a mounted sensor which may obstruct physical activity or may be uncomfortable for prolonged monitoring. Furthermore, contact PPG sensors experience a high degree of motion sensitivity which may corrupt the collected signal when the sensor-skin interface is disturbed.
PPG and, subsequently, pulse oximetry, are made possible due to a phenomenon known as an optical window where skin tissue exhibits relatively low absorption for visible and near-infrared light wavelengths between 400-2000 (nm). This property is primarily determined by the absorption spectra for water and skin pigments, particularly melanin. By contrast, blood and hemoglobin exhibit significantly higher absorption for wavelengths in the same region. As light enters the tissue, multiple complex interactions occur including absorption, reflection, scattering, transmission, and fluorescence, although absorption tends to be more important in the creation of an observed PPG signal. The total light absorption is a summation of a periodic AC signal associated with pulsatile blood volume changes and a low-frequency (to DC) offset component associated with absorption by the tissue, venous, and arterial blood. This AC signal component is created as heart contractions send pulsatile blood volumes to the peripheral vasculature that are observed as small, periodic changes in light absorption resulting from the varying blood volume in the underlying vasculature. The PPG signal may be measured in transmission mode, with the light source on the opposite side of the tissue as the sensor, or in reflection mode, where the light source and sensor are located on the same side of the tissue. Reflection mode is most common for imaging photoplethysmography (iPPG) applications where the photoplethysmogram is derived as a net intensity, over a region of interest, observed from reflected light that is captured by an imager.
Early work in the area of iPPG demonstrated pulsatile components with frequency characteristics (and their harmonics) similar to those of the photoplethysmogram (PPG) could be extracted from monochromatic imagery of a fingertip. This phenomenon was also observed in narrow-band wavelength imagery of a forearm at wavelengths of 660, 810, and 940 (nm), along with a lower frequency component related to respiration rate.
While monochromatic images composed of visible and NIR light or narrow-band wavelengths in the red and NIR range, undoubtedly inspired by traditional PPG and pulse oximetry methods were traditionally used, it was demonstrated that superficial arterial pathways in the neck (e.g., the carotid arteries) could be isolated in mid-wave infrared (MWIR) imagery, which also showed spectral peaks in a frequency-domain representation of the MWIR imagery data that were likely to be related to pulse rate. Near-infrared region bands were used despite very well documented evidence that oxyhemoglobin exhibits the most absorption for wavelengths in the green band, which is largely responsible for the pulsatile component of the PPG that links to cardiac cycle rhythms.
iPPG from facial imagery showed improved iPPG data quality by using red, green, and blue (RGB) channels from visible spectrum imagery as the channel input space to an independent component analysis (ICA) decomposition. This decomposition improved estimates of cardiac measures (as compared to fingertip PPG) beyond those achieved using only the green source channel (where the iPPG signal appears to be most prominent). By extracting inter-beat interval (IBI) time series from the iPPG component, strong correlations for features related to heart rate variability were shown as compared to fingertip PPG. Additional work focused on exploring practical issues of the methodology such as imager quality, frame rate requirements, facial ROI selection for improved channel space signal-to-noise ratio (SNR), and applied testing of iPPG in clinical environments.
However, one practical issue that has not yet been addressed is that of head motion artifact and its effect on pulse rate component recovery from the ICA (or other blind source separation approach) decomposition of the imager channel space. Contemporary methodologies either restrict head motion to a limited range or eliminate it completely from the design. With cardiac activity monitoring likely playing an important role in short-term operational evaluations and decision making processes, as well as long-term research in the areas of applied neuroscience and closed-loop decision aid systems, the potential user acceptance of a non-contact assessment methodology that is as good as, or better than, traditional contact methods in terms of data quality and robustness would be an invaluable step toward transitioning long-term, persistent physiological monitoring to day-to-day operations. Accordingly, there is a need in the art for improved PPG methodologies that are not as susceptible to artifacts from head or other movement of the subject.